Particle agglutination procedures are widely used for immunodiagnostic tests. These procedures employ particles coated with ligands, usually antigens, which bind to multivalent antiligands in a solution. Numerous different types of particles, including glass beads and latex particles, are used in these types of tests, the latex particle tests being generally illustrative of this technology.
Both direct and indirect latex agglutination tests are known, with direct tests used to detect antibodies in solution and indirect tests used to detect antigens in solution. In a typical direct latex agglutination test, latex particles coated with antigens are combined with a solution suspected of containing an antibody. If an antibody is present, it cross-links, or agglutinates, the latex particles, agglutination signaling the presence of the antibody. In a typical indirect latex agglutination test, both latex particles coated with antigen and a known quantity of an antibody which binds the antigen are combined together with a solution suspected of containing a target antigen which also binds with the antibody. If the target antigen is present, agglutination of the latex particles by the antibody is inhibited, with inhibition of agglutination signaling the presence of the antibody. See L. Bangs, Uniform Latex Particles, 51-58 (published by Seragen Diagnostics, Inc., P.O. Box 1210, Indianapolis, Ind. 46202). Through these two applications of the particle agglutination phenomenon, a broad variety of diagnostic tests are available.
The high cost of uniform latex particles and the difficulty of binding antigens to these particles have led to the development of numerous alternative agglutination tests. One approach is to use liposomes (or even cells) as the particle to be agglutinated. See, e.g., U.S. Pat. No. 4,668,638 to Janoff et al.; U.S. Pat. No. 4,636,479 to Martin et al.; U.S. Pat. No. 4,598,051 to Papahadjopoulos et al.; U.S. Pat. No. 4,564,599 to Janoff et al.; and U.S. Pat. No. 4,529,712 to Jou et al. These procedures require that the antigen be bound to some type of particle. Another approach is to combine a lipid antigen with an emulsion of lipid droplets, the lipid droplets serving as the particles to be agglutinated. See, e.g., U.S. Pat. No. 4,605,630 to Kung et al. at column 1, line 65 to column 2, line 16. This requires the use of a double phase solution (an emulsion) as a test reagent. Still another approach was described by Larsson P. O. and Mosbach, K., FEBS Letters 98, 333 (1979). In their scheme, the bifunctional nucleotide N.sub.2 N.sub.2 '-adipodihydrazido-bis-(N.sup.6 -carbonylmethyl-NAD), or Bis-NAD, was synthesized and added to a solution of the tetrameric enzyme lactate dehydrogenase (LDH), for which NAD is a strong ligand. The simultaneous binding of the bifunctional ligand to two different LDH molecules leads to aggregation and network formation of an enzyme-ligand complex. A disadvantage of this approach is that it is limited to the specific bifunctional ligand synthetic scheme disclosed.
Because of the importance of immunoassay procedures as diagnostic tests in the health care field and the limitations inherent in the procedures described above, there is a continuing need for simple and inexpensive agglutination tests. The present invention provides such a test.